Electrical protective system



May 8, 1956 D. E. GRAVES ETAI- 2,745,020

ELECTRICAL PROTECTIVE SYSTEM Filed Jan. e, 195o United States Patent @mee y 2,745,020 Patented May 8, 1956 ELECTRICAL PRoTEcrrvE sYsrEM Donald E. Graves and Dominic M. Cameli, Milwaukee,

Wis., assignors to General Electric Company, a corporation of New York Application .lanuary 6, 1950, Serial No. 137,194

Claims. (Cl. Z50-95) rhe present invention relates 'in general to systems for supplying power to electrical load devices, and has more particular reference to the provision of novel protection means for allowing the delivery of electric power to a load device only in amounts, insufficient to dangerously overload the device, during a selected operating time interval.

While the teachings of the present invention may be applied in any system for the supply of electrical power to a load of the sort operable for short intervals under conditions of substantial overload, the invention has particular application in power supply systems for X-ray generators, and like electrically energized devices, the operation of which at exceedingly high energy levels during short operating intervals, is desirable or necessary, but which, if operated continuously, or for too long an operating interval, at such high energy levels, may become irreparably damaged.

An X-ray generator commonly comprises a tube embodying an anode and a cooperating cathode enclosed in a sealed and evacuated envelope. Such a tube operates for X-ray production as the result of impingement, upon the anode, of electrons emitted by the cathode. During the operation of the tube for X-ray production, electrons emitted by and at the cathode are impelled toward and caused to impinge upon the anode, under the influence of electrical potential of suitable polarity maintained between the anode and cathode. The cathode comprises an electron emitting element, usually in the form of a lament adapted to be electrically energized or excited for electron emission.

During operation of the generator, heat is developed therein as the result of electron impingement, at high velocity, on the anode. The amount of operating heat thus generated is a function of the rate of electron impingement on the anode, which, in turn, is a function of the electron emission rate of the cathode, and also of the electron impelling voltage maintained between the anode and cathode for the operation of the device. The electron emitting rate of the cathode, in turn, is a function of cathode excitation, and may be expressed in terms of electrical current employed in exciting the cathode for electron emission.

The heat generated as the result of operating an X-ray tube may be of the order of the melting point of the anode metal, and it is commonly dissipated as rapidly as possible, from the anode, through the walls of the tube, or by the circulation of a cooling iluid in heat exchange relation with the anode, or by other heat dissipating means, the generator tube being ordinarily designed to dissipate operating heat therefrom, as rapidly as possible, within the heat transfer limits of the dissipating means. An X-ray generator, however, is usually operated during relatively short intervals of time, at relatively high energy input levels, especially where rays of intense penetrating character are required, ray intensity being also a function of the rate of electron impingement onvthe anode. Desirable operating energy levels may be ofan order such that, if continuously applied to the tube during an unduly extended operating interval, the temperature thereof, because of the limited heat dissipating ability of the tube, may increase above the melting point of its constituent parts, thereby damaging or destroying the X-ray tube.

An important object of the present invention is to pr'ovide means for limiting the amount of electrical energy that may be supplied to an electrical load device, to a quantity such that the device may not be'operated under dangerously overloaded conditions during a limited period for which it is setto operate; a further object being to provide apparatus which will allow the device to be operated, at any time, at a maximum input energy level just short of the safe heat absorbing capacity of the device during a limited operating interval.

Another important object is to provide protective means for preventing the delivery of electrical energy to an X-ray tube if the same be adjusted for operation, at a dangerously 'high energy level, during an unduly extended interval of operation; a further object being to allow operation of the device in any adjusted condition as to cathode emission rate, operating anode cathode potential, and operat- Y and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawing, discloses a preferred embodiment of the invention.

The single figure of the drawing is a diagrammatical representation of a protective system embodying the present invention.

To illustrate the invention, the drawing shows an adjustable load device comprising, in the present instance, an X-ray generator tube 11, including an anode 12 and an electron emitting cathode shown as a filament 13, adapted to be suitably energized for operation as a cathode. The anode and cathode may be conventionally enclosed in a sealed, evacuated envelope 14 and'electrically connected with a suitable source of energizing power. The X-ray generator tube l1 may be operated either as a selfrectifying device, by connecting the anode and cathode directly with a suitable source of alternating current power, or it may be operated by application of direct current power between the anode and cathode. As shown, the anode and cathode of the device 11 are interconnected with the direct current output of a suitable full wave rectifying system, comprising the rectiers 15 and energized by alternating current power throughl a suitable transformer 16. The secondary or high voltage windings 17 of this transformer are suitably connected with the input side of the full wave rectifying system, the primary Winding i8 of the transformer being connected with a source of relatively low voltage alternating current power, through instantaneous overload protective means hereinafter more particularly described.

The tube il may be operaed as an X-ray generator by energizing the cathode i3 for the emission of electrons and by simultaneously applying relatively high electric potential between the anode and cathode of the tube to impel cathode emitted electrons at high velocity upon the anode. For ypurposes of adjustment of the cathode electron emission rate, conventional means may be provided for energizing it at any selected one of a number of cathode exciting levels, and conventional means may 'the heat dissipating capacity of the tube.

also be provided for selectively adjusting the anode-cathodeop'era'ting voltage.

When an X-ray generating tube is operated as such, heat is released therein at a rate many times greater than lt is not only necessary to allow the tube to cool offl from time to time to prevent the same from reaching destructively high temperatures as the result of operation at too frequent intervals, but it is also necessary to limit the amount of energy delivered to the tube, during any sinfle time limited period of continuous operation, to an amount such that the tube, during the operating interval, may not reach a dangerously high temperature at the anode.

While a certain amount of heat is generated in the tube as the result of the release therein of cathode exciting energy, such heat is relatively insignificant when compared with the enormously larger quantities of heat developed at the anode as a result of X-ray producing electron impingement thereon. For practical purposes in determining the dangerous heat loading of a tube, operated during a limited time interval, cathode energizing heat may be disregarded. The cooling characteristics of the tube may also be disregarded, as they have relatively small effect during short operating time intervals. Thus it is that the dangerous heating effect, due to tube operation during limited, relatively short time intervals, may be measured, with sufficient exactness for practical purposes, as an approximation of the number of electrons impinging on the anode during the operating interval. This, as previously suggested, is a function of anode-cathode voltage, cathode exciting current, and the operating time interval; and the present invention proposes apparatus automatically adjustable, in accordance with the preselected operating interval during which the tube is to be operated, to limit the amount of electron impel'ling energy that may be applied between the anode and the cathode of the device, during such operating interval, whereby to prevent operation of the tube under dangerous overload conditions.

-To this end, operating energy may be delivered between f the anode and cathode of the tube through the transformer 16 from any suitable source of alternating current power 19, which may be connected, as through switch means 20 and overload protection devices 2l, to an auto transformer 22. The auto transformer may have a tap connection 23 connected with one side of the primary winding 18 of the transformer i6, as through a normally open switch 24. The auto transformer may-also be provided with an adjustable tap connection 25 electrically connectedwith the other side of the transformer winding 18, whereby anodecathode operating power, at potential determined by adjustment of the connection 25, may be supplied to the X-ray tube under the control of the switch 24, said switch being associated with a corresponding relay coil 26, adapted, when electrically energized, to close .its associated switch. The adjustable tap connection 25 thus comprises a selector switch for determining the operating voltage applicable between the anode and cathode of the X-ray tube 11, The relay coil l26 is connected in a corresponding actuating circuit, extending between tap connections 27 and 28, on the auto transformer, and controlled by a preferably single pole, double throw selector switch 29. The relay coil 26, on one side thereof, may be connected with the auto transformer tap 27', and may also be connected, through a pair of timer switches 30 and Stic, to a selecting contact of the switch 29, the pole of which is connected with the auto transformer tap 23 through a manually operable switch 31 and the normally closed switch 32 of a control relay 33.

A pair of timer switches 30' and 3de, similar to switches 3) and 3Go, may be connected in parallel relation with respect to the switch 24, between the primary 18 of the transformer 16 and the auto transformer tap 23. Consequently, anode-cathode operating power may be delivered, either under direct control of the timer switches 30 d and 30'c, or under control of the switch 24 indirectly controlled by the timer switches 3f? and 30C. lt will, of course, be obvious that the timer switches 30' and 30c could also be connected to indirectly control anode-cathode power, through a relay switch, like switch 24, .if desired.

It will be seen from the foregoing that, when and so long as the switches 31 and 32 are closed, the operating coil 26 of the switch 24 may be energized under the control of the timer switches 30 and 3de, depending upon the selectively adjusted condition of the switch 29. The timer switches 3i) and 30e respectively form operating parts of a synchronous timing mechanism 34. The timer switches 30 and 30c form like operating parts of an impulse timer mechanism 34. -The switches 30 and 30 are normally open switches, which are adapted to close when and so long as coils 35 and 35', associated therewith, are electrically energized. The switches 30C and Stlc are normally closed switches adapted to be opened by and during the operating cycle of the timing mechanisms in which the same are operating parts. These separate timing devices are adapted to be adjusted for operation so that, when released, they will operate to close their respective switches 30 and 30 and then open switches 30e and 306 after a predetermined time interval.

The synchronous timing mechanism 34 is adjustable to operate its associated switches 30 and 30C during intervals measured in terms of elapsed time. The impulse timer 34.- is adjustable to operate its associated switches 30' and 30'c during time intervals measured in terms of the number of energy impulses transmitted at predetermined frequency during the selected interval to be measured. Synchronous and impulse timing mechanisms of the sort herein contemplated are respectively illustrated and described in United States Patents No. 1,740,405 and No. 2,325,860.

Each of the timers 34 and 34' respectively includes a relay coil 35 and 35 operatively associated respectively with the switches 30 and 30', to close same when energized, and to release the adjusted mechanism for the performance of its timing function, said coils being connected between the taps 27 and 28 under control of the switches 29, 31, and 32. Accordingly,as determined by the selector switch 29, one or other of the coils 35 and 35 will be energized to close its associated switch 30 or 30 and start the corresponding timing device in operation, upon closure of the manual switch 31, providing, of course, that the switch 32 is then in closed condition. Such closure of a switch 30 or 30 will energize one o-r other of the coils 35 or 35 and operate one or other of the timer mechanisms to deliver operating power through the transformer 16 to the anode and cathode of the X-ray tube. As a result o-f the operation of the timing device, the switch Stic, or Stic, as the case may be, will be opened mechanically, upon the expiration of the tube operating interval for which the timing device shall have been previously adjusted, thereby disabling the supply of operating power in the anode-cathode circuit of the tube l1.

The overload protective system of the present invention comprises apparatus automatically adjustable, in response to adjustment of the timing mechanism, adjustment of the anode-cathode electron driving voltage supply, and adjustment of cathode excitation, to prevent operation of the X-ray generating tube lll in the event that such selective adjustments are made in manner which would result in dangerously overloading the X-ray generating tube 11 if, so adjusted, it were to be operated. When such a dangerous condition of adjustment occurs, the protective apparatus operates to open the exposure control circuit at the switch 32, thereby making it impossible to operatively energize the X-ray generating tube 11 by closure of the manual control switch 31. At the same time, a warning signal is caused to operate to indicate faulty adjustment of the apparatus.

To these ends, the relay`33, inr addition to thenormally closed switch 32, comprises a normally open switch 36, for controlling a warning device 37, which may conveniently comprise a buzzer or other audible or visual signalling means. The relay 33 also includes a switch operating coil 38, which, when energized, serves to open the switch 32 and close the switch 36. The relay coil 38 is connected in a series circuit between the anode and cathode of a thyratron tube 39, such series circuit preferably including a manual reset switch 40, a control resistor 41, and a source of direct current plate circuit power 42, in series with the coil 38. The thyratron tube has a control grid 391 normally held at substantial negative bias, with respect to the cathode, whereby the tube 39 normally remains nonconducting so that no current may flow in its plate circuit through the coil 38. A condenser 392 may be connected between the control grid 391 and the cathode of the thyratron tube 39 in order to absorb and thus eliminate the etect of voltage surges which otherwise might appear on the control grid 391 in response to the adjustment of the voltage selector switch 25. The tube may be triggered or fired to render it conducting, by reducing the negative -bias on the grid to the triggering voltage level, after which current may flow through the coil 38 and cause it to open the switch 32 and simultaneously to close switch 36, thus disabling the power supply means to the tube 11 and energizing the warning device 37. After thus becoming conductive, the tube 39 will continue in such condition, even after high negative bias on the grid is restored, unless the plate circuit of the tube is interrupted momentarily. This may be accomplished by opening the reset switch 40, for a moment, and then reclosing it.

Power for operating the protective system may comprise a transformer 43 having a primary winding 44 connected between the taps 27 and 28 of the auto transformer. The transformer 43 has a secondary winding 45 connected in series with the switch 36 and alarm device 37 to energize the same upon closure of the switch 36. The translformer winding 45 also may be connected to energize the cathode of the thyratron tube 39, preferably through a resistor 46. The transformer 43 also has a secondary winding 47 having a center tap 48 and interconnected with a preferably electronic rectifier tube 49. The tube 49 has a cathode, and plate means, said plate means being connected with the opposite sides of the transformer winding 47 for full wave rectification between the tap 48 and the cathode. The transformer 43 may have a separate secondary winding 5t? for energizing the cathode of the rectifier. A choke lter 51 and a capacitor 51 may be connected, as'shown, in the output side of the rectier, between the tap 43 and the cathode of the rectier tube 49; and a resistor S2 and voltage regulating tube S2 may also be connected in the rectie'r output to provide a constant,

rectified output voltage.

Means is provided for variably biasing the grid of the tube 39 in accordance with the time, cathode excitation,

and anode-cathode voltage adjustments of the X-ray apparatus, so that the thyratron will remain non-conducting, so long as such adjustments are within safe limits, but will become conducting as soon as the same become unsafe. To this end, the equipment for automatically adjusting the bias on the grid of the thyratron tube 39 may comprise a special tapered resistance 53 coupled mechanically with the synchronous timer 34 and adapted to be adjusted in acxcordance with the interval selecting adjustment of the timer. A similar tapered resistance 53' may be operative-- ly associated and mechanically connected, in like fashion, with the impulse timer 34. A switching relay 54 is provided for connecting the resistance elements 53 and 53' vselectively in circuit with the output side of the rectier adapted, when energized, to close the switches 55 and 56' and to'open switches 55' and 56. The resistors 53 and S3 iare arranged as potentiometers having each a sideconnected, respectively, through adjustable resistance means 5S and 58', with the cathode of the thyratron tube 39, yand A lthence through the resistor 4l and power source 42, to o-ne Aside of the rectiier output. The other sides of the potentiometer resistors 53 and 53' are connected, respectively, with the switches 56 and 56', and thence to the other side 48 of 'the rectifier output.

The adjustable members ofthe tapered potentiometer resistors 53 and 53' may be respectively connected with the relay switches 55 and 55 and thence with lthe grid of ythe .thyratron tube'39, through resistors 59 and 60. The coil 57 is connected in circuit between the auto transformer taps 27 and 28 through a switch adapted to close when the switch 29 is in its synchronous timer selecting position. Accordingly, the coil 57 will be connected between the auto transformer taps 27 and 28, and so energized, only ywhen the switch 29 is in adjusted position to select the -synchronous timer 34 for operation. At all other times the coilwill remain de-energized.v When the coil 57 is energized, only the tapered resistor 53 will be operatively v connected with the rectifier output and the control grid of the thyratron tube 39 by closure of the normally open vvswitches 55 and 56. Conversely, as long as the coil 57 remains de-energized, only the tapered resistor 53' will be operatively connected. Thus, the relay 54 serves yautomatically to connect the potentiometers 53 and 53' for bias control of the tube 39 as the switch 29 is in posiytion selecting timer 34 or 34 for operation.

It will be seen that the adjusted position of the movable member of the resistor 53 or 53', when operatively connected with the tube 39 and the rectier 49, will determine the amount of negative bias applied to the grid of the thyratron tube 39, such bias becoming progressively less negative and approaching the firing point of the thyratron f tube as the mechanically connected and associated timer mechanism is adjusted toward its longer time settings, whereby the tube may be caused to fire and operate the protective relay if the time setting is made for a dangerously extended interval. f

Conventional means, such as the lament transformer 16', the tapped autoformer 17, and the adjustable selector switch 18', may be provided 4for energizing the cathode 13 of the X-ray generator tube 11 at any selected one of ay number of cathode excitation levels determined bythe setting of the switch 18'; and means is provided for varying the bias on the grid of the thyratron tube in accordance with the adjustment of the excitation of the cathode of the X-ray tube inV accordance with the setting of the `switch 18', as well as the anode-cathode voltage adjustment accomplished by means of the switch 25. To this end, a plurality of potentiometers 61 may be provided to correspond, respectively, with the several cathode excitation settings at which `the X-ray tube may be selectively `adjusted. In this connection, the X-ray tube control equipment commonly includes cathode excitation control means operated by selector buttons, whereby the equipment may be conditioned to apply any desired cathode excitation to the X-ray tube by'pushing the appropriate selector button on .the control panel`of the apparatus, selector clearing means being provided to retract any previously pressed but- -ton to `inoperative position when a selected button is pushed. j

` The potentiometers 61 comprise resistance elements connected in parallel relationship between the auto transformer taps 23 and 25, and hence are energized at all times at a potential proportional to the operating potential applied between the anode and cathode of the Xeray tube. Each potentiometer 61 has an adjustable member connected through a corresponding, normally open switch 62 with the grid of the thyratron tube, through a capacitor 63 and preferably through the resistor 60. The switches 62 are each connected with a corresponding X-ray tube cathode excitation selector button. Accordingly, when and alsoincreases Withincreased cathode excitation.

the X-ray tube apparatus is conditioned tov provide cathode excitation: at a selected intensity, the potentiometer 61 corresponding Withthe selected degree of cathode excitation will be connected with the grid of the thyratron tube. Each potentiometer 61, of course, may be individually adjusted'by means of its adjustableielement to apply grid` bias on the tube in accordance Withthe degree of X-ray tube cathode excitation which it represents.

The basic negative bias applied' to the grid of the thyratron` through the potentiomete'rslSSror 53', as a function of the time setting, will thus be modified by a positive bias component supplied through a potentiometer selected from the potentiometers 6i, said component being a propor-tional function not'only of the' anode-cathode potential at which the X-ray tube control-apparatus is adjusted to operate, but also ofthe selected cathode excitation setting. The apparatus is arranged so that the positive bias comportent increases with increased anode-cathode voltage,

Accordingly, the protective system may be adjusted to tire the thyratron tube 39 and disable the X-ray tube power supply system whenever the combined time, anode-cathode voltage and cathode excitation* settings of the X-ray tube energizing, equipment areA such as would result inl dangerously overloading the X-ray tube should it be operated under such set conditions.

The potentiometers 53 and 53 are adaptedV to apply a maximum negative bias to thev grid of the tube 39, when the timers are set to their shortest interval timing adjustments. Such bias may be adjusted by means of the rheostats 58 and 5%. As a timer 34 or 34 is adjusted or set to longer X-ray exposure intervals, the corresponding adjustment of the potentiometer 53 or 53 will proportionally reduce the amount of negative bias on the grid of the tube 39. Since the potentiometers 61y are connected between the auto transformer taps 23 and 25, the bias component supplied to the grid of the tube 39, through the selected potentiometer di, will be of positive character, during positive half cycles of energy supplied from the source 19, and will be proportionaly to the X-ray tube anodecathode voltage, as determined by the adjustment of the tap 2,5'. The bias component'suppliedthrough the selected potentiometer 6l7 will also be proportional to the'selected X-ray tube cathode excitation, because of pre-set adjustment of the selected potentiometer. Since the bias component supplied through the selected potentiometer 61 is positive during alternate half cycles of energy supplied from the source 19, it will make the resultant bias, upon the grid of the tube, less negative during the intervals of such alternate half cycles. In the event that the resultant bias should reach the critical tube-ring'value, during such alternate half cycle intervals, because of the adjustment of the timer to a dangerously long exposure interval, in View of the anode-cathode voltage and cathode excitation adjustments, thetube 39 will become conductive, and the relay 33 will operate to'prevent the supply of operating power to the X-ray tube.

The system thus operates to apply a basic bias to hold the tube 39 normally inactive; to adjust the bias toward tube activating conditions as the timer is adjusted to longer time settings, and to further modify the bias toward tube activating condition in proportion to higher anode-cathode voltage and cathode excitation adjustments for the operation of the X-ray tube.

The capacitor 63 functions as a direct current blocking condenser and as a coupling capacitor to apply alternating voltage from the selected potentiometer 6l to the grid biasing circuit.

if desired, a normally closed disabling switch 64 may be connected in the energizing circuit of the relay switch coil 57. This switch 64 may be mechanically coupled and interlocked withy one or more of the switches 62' so that the switch 54 will open and remainl open solong as the switch or switches 62, 'with which: it isconnected, are in closedy position. This will prevent the equipment from being operated under synchronous timercontrol, at, cathodey excitation levels corresponding: with those of the' switches 62, which are thusy interlocked with the disconnect switch 64.

it is thought that the invention and its numerous attendant advantages will be fully understood fromy the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the' spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

l. A system for delivering power to an electron ow device, such as an X-ray tube, wherein heat generation is a function of several adjustable variables, including anode-cathode potential, cathode excitation and elapsed time of operating energy supply to the device, comprising a relay switch connected to control the supply ofoperating` energy to said device, an electron trigger valve having a control grid and adapted for operation at a predetermined grid bias level and connected to control said relay switch, means for variably biasing the control grid ot said valve in accordance with the adjustment of, one ot said adjustable variables, and means for modifying said bias inaccordance with the adjustment of said other adjustable variable.

2. A system for delivering power to an electron flow device, such as an X-ray tube, wherein heat generation is a function of several adjustable variables, including anode-cathode potential, cathode excitation and elapsed time of operating energy supply to thel device, comprising a relay switch connected to control the supply of operating energy to said device, an electron trigger valve have a control grid and adapted for operation*v at a predetermined grid bias level and connected to control said relay switch, meansfor variably biasing the control grid of said valve in accordance with the adjustment of said elapsed time variable, and means for modifying saidl bias in accordance with the adjustment of the anode-cathode potential and the cathode excitation level of said device.

3. A system for delivering power to an electron ow device, such as an X-ray tube, wherein heat generation is a function of several adjustable variables, including anode-cathode potential, cathode excitation and elapsed time ofoperating energy supply to the device', comprising a relay switch connected to control the supply of operating energy to said device, an electron trigger valve having a control grid and adapted for operation at a predetermined grid bias' level and connected tok control said relay switch, means adjustable with said velapsed time variable to negatively bias the controly grid of said valve increasingly below said triggering bias level as the elapsed time variable adjustment of the device is reduced, and means adjustable with the anode-cathode potential and cathode excitation variables for applying, on the grid of said valve, a bucking bias component of increasingly positive character as the anode-cathode potential and cathode excitation are increased.

4. A system for delivering power to an electron flow device, such as an X-ray tube, wherein heat generation 4is a function of several adjustable variables, including anode-cathode potential, cathode excitation and elapsed time of operating energy supply to the device, comprising a relay switch connected to control the supply of operating energy to said device, an electron trigger valve, adapted for operation at a predetermined negative bias level and connected to control said relay switch,. a' potentiometer adjustable with the elapsed operating time adjustment of the device to negatively bias said' valve increasingly below said triggering bias level as the elapsed time variable adjustment' of the deviceis reduced, and potentiometer means energized` in proportionto the. anodecath'ode potential' adjustment` and adjustable in accordadapted for operation at a predetermined negative bias level and connected to control said relay switch, a

potentiometer adjustable with the elapsed operating time g adjustment of the device to negatively bias said valve vincreasingly below said triggering bias level as the elapsed time variable adjustment of the device is reduced, and a plurality of potentiometers all energized in proportion to the anode-cathode potential adjustment7 and each ladjustable in accordance with a corresponding one of a j number of cathode excitation levels to which the device may be vselectively adjusted, for applying a bucking bias component, on said valve, of increasing positive character vas the anode-cathode potential and cathode excitation are increased.

6. A systeml for delivering power to an electron flow device, such as an X-ray tube, wherein heat generation is a function of several adjustable variables, including anode-cathode potential, cathode excitation 'and elapsed time of operating energy supply to thedevice, comprising a relay switch connected to control the supply of operating energy to, said device, an electron trigger valve, adapted for operation at a predetermined negative bias level and connected to control said relay switch, a potentiometer adjustable with the elapsed operating time adjustment of the device to negatively bias said valve increasingly below said triggering bias level as the elapsed time variable adjustment of the device is reduced, and a plurality of potentiometers all enerigized in proportion to the anodecathode potential adjustment, and each adjustable in accordance with a corresponding one of a number of cathode excitation levels to which the device may be selectively adjusted, for applying a bucking bias component, on vsaid valve, of increasing positive character as the anode-cathode potential and cathode excitation are increased, and interlocking switch means, operable upon adjustment of the device for operation at one of said cathode excitation levels for preventing the delivery of operating power to said device. n

7. The combination, with an X-ray tube having an anode and an electron emitting cathode, of relay switch means for controlling the delivery of operating power between said anode and cathode, adjustable means for bias in accordance with the adjustment ot said other adjustable means.

8. The combination, with an X-ray tube having an anode and an electron emitting cathode, of relay switch means for controlling the delivery of operating power between said anode and cathode, adjustable means for varying the anode-cathode potential at which said power is to be delivered, adjustable means for exciting the cathode for electron emission, at various intensity levels, adjustable timer means for determining the elapsed time interval during which operating power may be delivered to lthe tube, any electron trigger val-vev adapted for operation at a predetermined bias level and controllingly connected with said relay switch means, means' adjustable with said timer means to negatively bias said valve increasingly below said triggering bias level as said timer means is adjusted progressively for shorter elapsed time operation, and means operable in accordance with said anode-cathode potential adjustment, and in accordance with said adjustment of cathode excitation, to apply, on said valve, a bucking bias component of increasingly positive character in proportion to increase in adjusted anode-cathode potential and cathode excitation intensity.

9. The combination, with van electrical load device, in which the generation of heat is a function of load current, load voltage and the elapsed time during which electrical power is applied to the load device, of relay switch means for controlling power delivery to said load adjustable means for varying load voltage, adjustable means for varying load current, adjustable timer means for determining the time interval during which operating power may be supplied toy said load device, an electron trigger valve having a control grid and adapted for operation at a predetermined grid bias level and controlling' connected with said relay switch means, means adjustable with saidtimer means to negatively bias the grid of said valve increasingly below its triggering bias level as said timer means is adjusted progressively for shorter elapsed time operation, and means operable in accordance with the adjustment of load voltage 'and' of load current to apply, on the grid of said valve, a bucking bias component of increasingly positive character in proportionv to increase in adjusted load voltage and current.

l0. A system for delivering powerto a heat generating electrical load in which the generation of heat is a function of several adjustable variables, including load voltage, load curr/ent and the elapsed time interval during which electrical power is applied to the load, comprising a relay switch connected to control the supply of power to said load, an electron trigger valve having a control grid and adapted for operation at a predetermined grid bias level and connected to control said relay switch, means for applying a bias on the grid of said valve varying in accordance with the adjustment of said elapsed time variable, and means for modifying said bias in accordance with the adjustment of load voltage and current.

ll. A system for delivering power to a heat generating electrical load in which the generation of heat is a function of a plurality of adjustable variables, comprising a circuit for supplying power to said load, a relay switch in said circuit for controlling the flow of operating powery to said load, an electron trigger valve having a control grid and adapted for operation at a predetermined grid bias level and connected with said switch to operate the same,means comprising a potentiometer, adjustable in accordance with the adjustment of one of said adjustable variables, to correspondingly bias the grid of said valve, and bias modifying means, adjustable in accordance with the adjustment of another of said variables, to correspond- 'ingly modify the bias on said grid.

12. A system for delivering power to a heat generating electrical load in which the generation of heat is a function of a plurality of adjustable variables, comprising a circuit for supplying power to said load, a relay switch in said circuit for controlling the flow of operating power to said load, an electron trigger valve having a control grid and adapted for operation at a predetermined grid bias level and connected with said switch to operate the same, means comprising a potentiometer energized at a voltage varying in accordance with the adjustment of one of said adjustable variables, to correspondingly bias the grid of said valve, and means to adjust said potentiom- V eter in accordance with the adjustment of another of said variables, whereby to apply a composite bias on said grid adjusted in accordance with the adjustment of both of said variables.

afname 13; Asys'tem for delivering power to a heat generating electrical'load in which the generation of heat is a function of plurality of adjustable variables, comprising an electron trigger valve having a control grid' and adapted for operation at a predetermined grid bias level and connected to control the ow of operating power to said load, means for applying a control bias on the grid of said valve varying in accordance with the adjustment of one of said adjustable variables, and means for modifying said bias in accordance with the adjustment of another of said Variables.

14. A system for delivering power to a heat generating electrical device inwhich thegeneration of heat is a functionof a plurality of variably adjustable operating conditionsl includingduration of operating time inter-valv and operating voltage at which power may be delivered, which conditions may be selectively determined and adjusted prior to' the `placement of the device in operation, comprising a normally closed switch for controlling the delivery of power to said device, a normally open switch closable to set the device in operation` under selected operating conditions, an electron trigger valve having a control gridl and adapted for operation at a predetermined gridv bias level to open said normally closed switch, means for applyinga control bias of predetermined value on the grid of said valve to hold the same normally inoperative, means operable to alter said bias in accordance with the adjustment of one of said adjustably Variable conditions, and means for modifying said bias in accordance with the adjustment of another of said adjustably variable` conditions, whereby said valve may be actuated to open said normally closed switch to thereby prevent delivery of power to said device in response to the closure of. saidlv normally open switch when said selected operating conditions are such that the device would be electrically overloaded if operated under such selected variable conditions.

l5. A system for delivering power to an electron flow device, such as an X-ray tube, wherein heat generation is a function of several adjustably variable operating conditions, including anode-cathode potential, cathode excitation level, and duration of operating time -interval,.whch conditions may be selectively determined and adjusted prior to the placement of the device in operation, comprising ay normally closedrelay switch for controlling the delivery of power to said device, a normally openswitch closable to set the device in oper-ation under selected operating conditions, an electron trigger valve having a control grid and adapted: for operation at a predetermined grid bias level to open` said normally closed switch, means for applying a control bias of predetermined value on the grid of said valve to hold the same normally inoperative, means operable to alter said bias in accordance with the adjustment of the operating time interval, and means for modifying said bias in accordance with the selected anode-cathode potential and also in accordance with the selected cathode excitation level, whereby said valve may be actuated to open said normally closed switch to thereby prevent the delivery of power to said device in response to the closure of said normally open switch when said selected values of operating potential, cathode excitation, and duration of operating time interval are such that the device would be electrically overloaded4 if operated under such preselected operating conditions.

References Cited in the leof this patent UNITED STATES PATENTS 2,339,902 Akers et al Ian. 25, 1,944k 2,353,979 Weisglass July 181944 2,379,125 Weisglass June 26, 1945 2,384,647 Schwarzchild Sept. 11, 1945 2,572,258 Goldeld et al Oct. 23, 1951 2,641,701 Moore June 9, 1953 

